Tunable laser source with integrated optical modulator

ABSTRACT

A tunable laser source with integrated optical modulator. The tunable laser source is a widely tunable semiconductor laser that is comprised of an active region on top of a thick, low bandgap, waveguide layer, wherein both the waveguide layer and the active region are fabricated between a p-doped region and an n-doped region. An electro-absorption modulator is integrated into the semiconductor laser, wherein the electro-absorption modulator shares the waveguide layer with the semiconductor laser.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. Utility patent application Ser.No. 10/049,362, filed Feb. 6, 2002, by Thomas G. B. Mason, Larry A.Coldren, and Gregory Fish, entitled “TUNABLE LASER SOURCE WITHINTEGRATED OPTICAL MODULATOR,” which application claims priority under35 U.S.C. §119(e) of U.S. Provisional Patent Application Ser. No.60/152,432, filed Sep. 3, 1999, by Thomas G. B. Mason, Larry A. Coldren,and Gregory Fish, entitled “TUNABLE LASER SOURCE WITH INTEGRATED OPTICALMODULATOR,” both of which applications are incorporated by referenceherein.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT

This invention was made with Government support under Grant No.N00014-96-1-6014, awarded by the Office of Naval Research. TheGovernment has certain rights in this invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates in general to semiconductor lasers, and inparticular to a tunable laser source with integrated optical modulator.

2. Description of the Related Art

Modern day usage of optical components and lasers has madecommunications and data transfer more efficient and more cost effective.The use of semiconductor lasers has made the fabrication and packagingof optical sources more cost effective, as well as reducing the size ofthe overall device.

However, the requirements for communications and data transfer systemshave also increased. Widely tunable lasers are essential components fora wide variety of wavelength-division multiplexing (WDM) and packetswitching network architectures. They can be used as replacement sourcesin long haul dense WDM communication systems or for wavelength routingin access networks. They are also important devices for next generationphased array radar systems that use true-time delay beam steering.

In order to achieve a wide tuning range, these devices require fairlylarge passive tuning elements. This makes the devices four to five timeslarger than conventional fixed wavelength lasers. However, having thislarge amount of passive material in a laser cavity reduces the speedwith which they can be turned on and off by direct current modulation.Moreover, the rate at which they are able to transmit data is limited,making them unsuitable for high bandwidth applications.

There are two other factors that make it difficult to use these devicesto transmit data. The wavelength in a sampled grating distributed Braggreflector (SGDBR) laser is controlled by aligning a pair of reflectionpeaks in two mirrors with an optical cavity mode. When a gain current ismodulated over a wide range of currents, it can disturb this alignment,resulting in mode instability within the device, which is highlyundesirable for data transmission. To prevent this mode instability,such devices can only be modulated over a narrow range of output powers,which introduces a significant extinction ratio penalty to their datatransmission performance.

The other problem with directly modulating a laser is frequency chirp,which is the shift in the laser oscillation frequency that occurs whenthe output power level is changed. This is undesirable in transmissionsystems, since frequency chirp causes pulse spreading, which limits themaximum distance over which data can be sent over an optical fiber orother dispersive medium.

The three most successful types of widely tunable lasers are the superstructure grating distributed Bragg reflector laser (SSGDBR), thegrating assisted codirectional coupler with sampled grating reflectorlaser (GCSR), and the sampled grating DBR laser (SGDBR). All of thesedevices are capable of continuous tuning ranges greater than 40 nm.However, SGDBR lasers and other widely tunable designs have long activesections and fairly large optical cavities that limit their directmodulation bandwidth to between 3 and 4 GHz. This enables them to beused in OC-48 data transmission systems under direct modulation, if somewavelength chirping can be tolerated. However, this bandwidth isinsufficient for use in most phased array radar systems or in OC-192data transmission networks operating at 10 Gb/s.

In these applications, external modulators are frequently used to applya radio frequency (RF) signal or data to the optical carrier. Even forlong-haul OC-48 systems, external modulators are frequently used tominimize frequency chirp. However, external modulators add significantcost and complexity to the optical assembly which can be prohibitive insystems that require a large number of tunable lasers and modulators.For this reason, it is desirable to monolithically integrate a highspeed modulator with a tunable laser on as a single semiconductordevice.

SUMMARY OF THE INVENTION

To minimize the limitations in the prior art described above, and tominimize other limitations that will become apparent upon reading andunderstanding the present specification, the present invention disclosesa device, method, and article of manufacture related to a tunable lasersource with integrated optical modulator. The tunable laser source is awidely tunable semiconductor laser that is comprised of an active regionon top of a thick, low bandgap, waveguide layer, wherein both thewaveguide layer and the active region are fabricated between a p-dopedregion and an n-doped region. An electro-absorption modulator isintegrated into the semiconductor laser, wherein the electro-absorptionmodulator shares the waveguide layer with the semiconductor laser.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the drawings in which like reference numbers representcorresponding parts throughout:

FIG. 1 is a perspective view that illustrates the structure of a widelytunable semiconductor laser with an integrated electro-absorptionmodulator according to a preferred embodiment of the present invention;

FIG. 2 illustrates a cross-section of the laser and modulator accordingto the preferred embodiment of the present invention;

FIGS. 3A and 3B are charts that illustrate the tuning curves for widewavelength tuning according to the preferred embodiment of the presentinvention;

FIGS. 4A, 4B, and 4C illustrate the Franz-Keldysh effect in themodulator according to the preferred embodiment of the presentinvention;

FIG. 5 is a chart that illustrates the response curves for initial testsof the integrated laser and modulator device over a 50 nm tuning rangeaccording to the preferred embodiment of the present invention; and

FIGS. 6A and 6B are flowcharts illustrating the steps used in thefabrication process according to the preferred embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

In the following description of the preferred embodiment, reference ismade to the accompanying drawings which form a part hereof, and in whichis shown by way of illustration a specific embodiment in which theinvention may be practiced. It is to be understood that otherembodiments may be utilized and structural changes may be made withoutdeparting from the scope of the present invention.

Overview

The present invention provides a simple and effective method forcreating a tunable laser with an integrated optical modulator that canbe fabricated on a single semiconductor chip. The laser can be rapidlytuned over a wide range of wavelengths, thereby enabling it to be usedin a variety of applications from wavelength division multiplexed fiberoptic communications to phased array radar. Integrating the modulatorwith the laser provides a highly desirable method for modulating theintensity of the output from the laser without perturbing its modestability or introducing high levels of frequency chirp. It also enablesmuch higher modulation frequencies to be reached than with the laseralone.

Device Structure

FIG. 1 is a perspective view that illustrates the structure of a widelytunable semiconductor laser with an integrated electro-absorptionmodulator according to a preferred embodiment of the present invention.The laser 10 is a four-section buried-ridge sampled-grating distributedBragg reflector (SGDBR) laser. The four separate sections of the laser10 comprise tuning sections and include: a sampled grating back mirrorsection 12, a phase control section 14, a gain section 16, and a sampledgrating front mirror section 18. An electro-absorption (EA) modulator 20shares a common waveguide 22 with the back mirror section 12, phasecontrol section 14, gain section 16, and front mirror section 18 of thelaser 10, wherein the waveguide 22 is designed to provide high indextuning efficiency in the laser 10 and good reverse bias extinction inthe modulator 20. Generally, a bias voltage is connected to the top ofthe device and a ground is connected to the bottom. When the biasvoltage on the gain section 16 is above a lasing threshold, laser 10output is produced.

FIG. 2 illustrates a cross-section of the integrated laser 10 andmodulator 20 device according to the preferred embodiment of the presentinvention. The device is comprised of an n-doped region 24, a thick, lowbandgap waveguide layer 26 (to form the waveguide 22 shown in FIG. 1),an anti-reflective coating 28, a stop etch layer 30, an active region32, a p-doped region 34, a p+ contact layer 36, and metal contacts 38.

N-doped region 24 and p-doped region 34 are typically of differentdopant types. Although indium phosphide (InP) is used in the preferredembodiment, the doped regions 24 and 34, and active region 32 can be ofany laser producing material. Moreover, the relative positions of then-doped region 24 and p-doped region 34 can be reversed withoutdeparting from the scope of the invention.

A lateral waveguide 22 is formed by laterally patterning the waveguidelayer 26 or layer 34 above it into a stripe geometry. A key designelement of the present invention is that a single common waveguide 22 isused for the tuning sections 12, 14, 16, and 18 in the laser 10, and forthe modulator 20. The lateral waveguide 22 structure may be formed as aburied ridge, or other type of lateral waveguide 22 structure, such as asimple ridge.

The p-doped region 34 also includes proton implants 40 above and besidethe waveguide 22 that are blocking junctions that act as isolators toblock lateral current leakage in the laser 10, as well as to provideisolation between sections in the laser 10, to reduce parasitic junctioncapacitance of the modulator 20, and to provide isolation from themodulator 20.

The active region 32 includes offset multiple quantum wells (MQW) thatprovide the laser 10 output. When an electrical current passes from thep-doped region 34 to the n-doped region 24 through the active region 32,a laser 10 light output is produced from the MQW. The active region 32and the waveguide layer 26 are separated by the stop etch layer 30 toenable the active region 32 to be removed with a selective wet etchantduring fabrication.

Both the back and front mirror sections 12 and 18 comprise a set ofperiodically sampled gratings that are etched into the waveguide layer26 with a period 42. Gratings of this type act as wavelength selectivereflectors, wherein one or more specified sampling periods 42 willprovide a partial reflection at periodic wavelength spacings of anoptical signal carried by the waveguide 22. The laser 10 can be rapidlytuned over a wide wavelength range by proper adjustment of controlcurrents for the mirror sections 12 and 18, for example, as described inU.S. Pat. No. 4,896,325, which is incorporated by reference herein.

The anti-reflective coating 28 is applied to each end of the deviceafter the device is cleaved out of the wafer.

Tuning Curves

FIGS. 3A and 3B are charts that illustrate the tuning curves for widewavelength tuning according to the preferred embodiment of the presentinvention. In FIG. 3A, the X-axis is the back mirror 12 current (mA) andthe Y-axis highest wavelength (nm). In FIG. 3B, the X-axis is the frontmirror 18 current (mA) and the Y-axis highest wavelength (nm).

As shown in FIGS. 3A and 3B, the principal advantage of this laser 10 isthat it can be rapidly tuned over a wide wavelength range by properadjustment of control currents for the mirrors 12 and 18. This is ahighly desirable feature that makes the device of the present inventionuseful for current and next generation fiber networks.

Franz-Keldysh Effect

FIGS. 4A, 4B, and 4C illustrate the Franz-Keldysh effect in themodulator 20 according to the preferred embodiment of the presentinvention. In these diagrams, E₁is the energy level of the valence band(E_(v)), E₂ is the energy level of the conduction band (E_(c)) Δ E isthe bandgap between E₁and E₂, and Δ x is the thickness of the waveguidelayer 26.

In the present invention, performance is optimized by using a thick, lowbandgap waveguide layer 26. This provides good index tuning efficiencyin the mirror sections 12 and 18, and a reasonable extinction ratio andchirp parameter in the modulator 20.

The operation of the modulator 20 is based on either the Franz-Keldysheffect in a bulk semiconductor waveguide 22 or the quantum confinedStark effect in a MQW. When a strong electric field is applied to thewaveguide 22, the band edge of the material is shifted to lower energiesallowing it to absorb the output light of the laser 10, as shown inFIGS. 4A, 4B, and 4C. This technique allows very rapid modulation of thelaser 10 with minimal wavelength chirping. Under proper conditions, thiscan produce sufficient optical loss to extinguish the output lightintensity by more than 20 dB, even over a wide wavelength range.

For narrower ranges of operation, the modulator 20 could comprise an MQWmodulator grown into the center of a higher bandgap waveguide layer 26.This would provide less efficient tuning in the laser 10, but wouldallow for lower voltage operation in the modulator 20, since the bandgapdetuning can be reduced due to the sharper absorption edge of the MQWstructure.

Response Curves

FIG. 5 is a chart that illustrates the response curves for initial testsof the integrated laser 10 and modulator 20 device over a 50 nm tuningrange according to the preferred embodiment of the present invention.These initial tests of the buried heterostructure electro-absorptionmodulator 20 integrated with a sampled grating distributed Braggreflector laser 10 were conducted to demonstrate the operation andbenefits of the present invention. In these tests, a 400 nanometer thickwaveguide 22 with a bandgap wavelength of 1.4 microns was used. Thelaser 10 had a tuning range of more than 47 nanometers. The modulator 20was able to produce more than 26 dB over this entire tuning range withonly a 4 volt bias.

Fabrication Process

FIGS. 6A and 6B are flowcharts illustrating the steps used in thefabrication process according to the preferred embodiment of the presentinvention. FIG. 6A shows the steps used when a lateral waveguide isbeing formed as a simple ridge, while FIG. 6B shows the steps used forother types of lateral waveguiding structures, such as a buried ridgestripe.

Referring to FIG. 6A, Block 44 represents a first growth step, with ann-InP buffer layer 24, Q-waveguide layer 26, stop etch layer 30, activeMQW region 32, and small portion of the p-InP cladding layer 34 beinggrown.

Block 46 represents the patterning and selectively etching off of thethin top p-InP cladding layer 34 and active regions 32 down to thestop-etch layer 30, everywhere except in the gain section 16 of thelaser 10.

Block 48 represents the patterning and etching of one or more sets ofperiodically sampled gratings with period 42 in the mirror sections 12and 18.

For the simple ridge, Block 50 represents the second growth thatcompletes the vertical structure growth of the p-InP cladding layer 34and the p+ contact layer 36 beneath the contacts 38.

Block 52 represents the proton implants 40 being performed to isolatesections of the laser 10, and to provide isolation between the modulator20 and the laser 10.

Block 54 patterns the contacts 38, removes the p+ contact layer 36therebetween by etching, and metalizes the contacts 38.

Block 56 etches a ridge waveguide 22 stripe down to the active region 32or the waveguide layer 26.

Block 58 cleaves the laser 10 and modulator 20 device out of the wafer,and then applies antireflective (AR) coatings 28 on each end of thedevice.

Referring to FIG. 6B, Block 60 represents a first growth step, with ann-InP buffer layer 24, Q-waveguide layer 26, stop etch layer 30, activeMQW region 32, and thin p-InP cladding layer 34 being grown.

Block 62 represents the patterning and selectively etching off of thethin top p-InP cladding layer 34 and active regions 32 down to thestop-etch layer 30, everywhere except in the gain section 16 of thelaser 10.

Block 64 represents the patterning and etching of one or more sets ofperiodically sampled gratings of period 42 in the mirror sections 12 and18 that are coupled to the active region 32.

For the buried ridge, Block 66 represents the second growth step, whichcomprises only a thin additional growth of the p-InP cladding layer 34to cover the periodically sampled 42 gratings in the mirror sections 12and 18.

Block 68 represents a patterning and etching of the buried ridgewaveguide 22 stripe, which is etched laterally to beneath the waveguidelayer 26.

Block 70 represents a third growth of the p-InP cladding layer 34 andthe p+ contact layer 36.

Block 72 represents proton implants 40 being performed to isolatesections of the laser 10, and between the modulator 20 and the laser, aswell as to limit lateral current leakage.

Block 74 patterns the contacts 38, removes the p+ contact layer 36therebetween by etching, and metalizes the contacts 38.

Block 76 cleaves the laser 10 and modulator 20 device out of the wafer,and then applies antireflective (AR) coatings 28 on each end of thedevice.

CONCLUSION

This concludes the description of the preferred embodiment of theinvention.

In summary, the present invention discloses a device, method, andarticle of manufacture related to a tunable laser source with integratedoptical modulator. The tunable laser source is a widely tunablesemiconductor laser that is comprised of an active region on top of athick, low bandgap, waveguide layer, wherein both the waveguide layerand the active region are fabricated between a p-doped region and ann-doped region. An electro-absorption modulator is integrated into thesemiconductor laser, wherein the electro-absorption modulator shares thewaveguide layer with the semiconductor laser.

The foregoing description of the preferred embodiment of the inventionhas been presented for the purposes of illustration and description. Itis not intended to be exhaustive or to limit the invention to theprecise form disclosed. Many modifications and variations are possiblein light of the above teaching. It is intended that the scope of theinvention be limited not by this detailed description, but rather by theclaims appended hereto.

1. A method for fabricating a tunable laser with an integratedmodulator, comprising: (a) performing a first growth step, wherein abuffer layer, a waveguide layer, a stop etch layer, an active region,and a terminating layer are grown on a semiconductor wafer; (b)patterning and etching the terminating layer and active region down tothe stop-etch layer, everywhere except in a gain section of the laser;(c) patterning and etching of one or more sets of periodically sampledgratings in one or more mirror sections of the laser; (d) performing asecond growth step that completes a vertical structure growth of theterminating layer and a contact layer beneath one or more contacts ofthe laser; (e) isolating sections of the laser from one another, andbetween the modulator and the laser; (f) patterning the contacts of thelaser, removing the contact layer therebetween by etching, andmetalizing the contacts; (g) patterning and etching a ridge waveguidestripe down to either the active region or the waveguide layer and; (h)cleaving the device out of the wafer, and then applying anantireflective coating on at least one end of the device.
 2. The methodof claim 1, wherein the laser includes a sampled grating back mirror, aphase control section, a gain section, and a sampled grating frontmirror.
 3. The method of claim 1, wherein the waveguide layer is asingle common waveguide layer used for the sampled grating back mirror,phase control section, gain section, sampled grating front mirror, andmodulator.
 4. The method of claim 1, wherein the waveguide layer isdesigned to provide high index tuning efficiency in the laser and goodreverse bias extinction in the modulator.
 5. The method of claim 1,wherein the waveguide is a buried heterostructure waveguide thatincludes offset multiple quantum wells (MQW) that provide the laser'soutput.
 6. The method of claim 1, wherein the waveguide is a ridgewaveguide that includes offset multiple quantum wells (MQW) that providethe laser's output.
 7. The method of claim 1, wherein the waveguidelayer includes one or more blocking junctions that blocks lateralcurrent leakage in the laser and reduces parasitic junction capacitanceof the modulator.
 8. The method of claim 1, wherein the laser is rapidlytunable over a wide wavelength range by proper adjustment of controlcurrents for its mirrors.
 9. An article of manufacture comprising atunable laser with integrated optical modulator fabricated according tothe method of claim
 1. 10. A method for fabricating a tunable laser withan integrated modulator, comprising: (a) performing a first growth step,wherein a buffer layer, a waveguide layer, a stop etch layer, an activeregion, and a cladding layer are grown on a semiconductor wafer; (b)patterning and selectively etching off of the cladding layer and activeregion down to the stop-etch layer, everywhere except in a gain section;(c) patterning and etching of one or more sets of periodically sampledgratings in one or more mirror sections of the laser; (d) performing asecond growth step, wherein an additional growth of the cladding layeris grown to cover one or more periodically sampled gratings in themirror sections; (e) patterning and etching a buried ridge waveguidestripe, wherein the waveguide strip is etched laterally to beneath thewaveguide layer; (f) performing a third growth step that completes avertical structure growth of the terminating layer and a contact layerbeneath one or more contacts of the laser; (g) isolating sections of thelaser from one another, and between the modulator and the laser; (h)patterning the contacts of the laser, removing the contact layertherebetween by etching, and metalizing the contacts; (i) cleaving thedevice out of the wafer, and then applying antireflective coatings on atleast one end of the device.
 11. The method of claim 10, wherein thelaser includes a sampled grating back mirror, a phase control section, again section, and a sampled grating front mirror.
 12. The method ofclaim 10, wherein the waveguide layer is a single common waveguide layerused for the sampled grating back mirror, phase control section, gainsection, sampled grating front mirror, and modulator.
 13. The method ofclaim 10, wherein the waveguide layer is designed to provide high indextuning efficiency in the laser and good reverse bias extinction in themodulator.
 14. The method of claim 10, wherein the waveguide is a buriedheterostructure waveguide that includes offset multiple quantum wells(MQW) that provide the laser's output.
 15. The method of claim 10,wherein the waveguide is a ridge waveguide that includes offset multiplequantum wells (MQW) that provide the laser's output.
 16. The method ofclaim 10, wherein the waveguide layer includes one or more blockingjunctions that blocks lateral current leakage in the laser and reducesparasitic junction capacitance of the modulator.
 17. The method of claim10, wherein the laser is rapidly tunable over a wide wavelength range byproper adjustment of control currents for its mirrors.